Production of barium titanate ceramics having a positive temperature coefficient of electrical resistance

ABSTRACT

A process for improving the electrical properties of barium titanate ceramics having a positive temperture coefficient of electrical resistance comprises providing barium titanate ceramics having a positive temperature coefficient of electrical resistance, coating said barium titanate ceramics with at least one compound selected from the group consisting of silver compound and lithium compound; and heating the coated barium titanate ceramics at a temperature of 700*C to 1,200*C.

United States Patent 1191 Hirose et a1.

PRODUCTION OF BARIUM TITANATE CERAMICS HAVING A POSITIVE TEMPERATURECOEFFICIENT OF ELECTRICAL RESISTANCE Inventors: Namio Hirose; YoshihiroMatsuo;

Shigeru Hayakawa, all of Osaka, Japan Matsushita Electric Industrial Co.Ltd., Osaka, Japan Filed: June 9, 1972 Appl. No.: 261,330

Assignee:

US. Cl 117/224, 117/8, 117/64 R, 117/113, 117/223, 252/629, 310/8 Int.Cl B44d 1/02, B44d 1/06 Field of Search 310/8; 252/629; 117/223,117/224, 201,113,114 R, 8, 64 R References Cited UNITED STATES PATENTS6/1971 Muller 310/8 Primary Examiner Leon D. Rosdol AssistantExaminer-Michael F. Esposito Attorney, Agent, or Firm-Wenderoth, Lind &Ponack [57] ABSTRACT A process for improving the electrical propertiesof barium titanate ceramics having a positive temperture coefficient ofelectrical resistance comprises providing barium/titanate ceramicshaving a positive temperature coefficient of electrical resistance,coating said barium titanate ceramics with at least one compoundselected from the group consisting of silver compound and lithiumcompound; and heating the coated barium titanate ceramics at atemperature of 700C to 1,200C. g

2 Claims, N0 Drawings PRODUCTION OF BARIUM TITANATE CERAMICS HAVING APOSITIVE TEMPERATURE COEFFICIENT OF ELECTRICAL RESISTANCE This inventionrelates to the production of barium titanate ceramics having a positivetemperature coefiicient of electrical resistance (PTC barium titanatethermistor), particularly to the production of barium titanate ceramicshaving a large positive temperature coefficient of electricalresistance.

It has been well known that barium titanate has a perovskite structureand becomes semiconductive with the addition of rare earth, niobium,bismuth and antimony ions. semiconductive barium titanate ceramics havea positive temperature coefficient (PTC) of electrical resistance abovetheir Curie temperature, and the substitution of lead (Pb) and tin (Sn)for barium (Ba) and titanium (Ti) respectively changes their Curietemperature.

The application of PTC barium titanate thermistor for electrical devicesrequires a PT C barium titanatethermistor having a large positivetemperature coefficient of electrical resistance (R /R R maximumresistance, R minimum resistance) to control large electric powers.

it is an object of this invention to provide a process for making PTCbarium titanate thermistor having a large positive temperaturecoefficient of electrical resistances (R /R This object will becomeapparent upon consideration of the following detailed description.

A process for improving the electrical properties of barium titanateceramics having a positive temperature coefficient of electricalresistance according to the present invention comprises providing bariumtitanate ceramics having a positive temperature coefficient ofelectrical resistance, coating said barium titanate ceramics with atleast one compound selected from the group consisting of silver compoundand lithium compound and heating the coated barium titanate ceramics ata temperature of 700C to 1,200C.

The barium titanate ceramics referred to herein are in a compositioncomprising, as a main component, titanium dioxide and barium carbonateand an effective additive to cause a positive temperature coefficient ofelectrical resistance. The effective additives are niobium oxide,bismuth oxide, antimony oxide, and rare earth oxide as shown in manyliteratures. If necessary for control of Curie temperature, the bariumtitanate ceramics may be modified with any effective element such aslead for barium and tin for titanium.

The silver compound or lithium compound referred to hereinis a materialto be converted into silver oxide or lithium oxide upon being heated ata temperature trate, silver carbonate, silver oxide, silver sulfide,silver sulfate, silver sulfite, silver fluoride, lithium nitrate,lithium carbonate, lithium oxide, lithium oxalate and lithium hydroxide.

The barium titanate ceramics can be coated with the silver compound orthe lithium compound by any suitable and available method. For example,the barium titanate ceramics in a crusible can be covered with thesilver compound and/or lithium compound in a powder form. A preferablemethod is to immerse the barium titanate ceramics in an aqueous solutionof the silver nitrate and/or lithium nitrate and dry the barium titanateceramics. The operable aqueous solution is in a concentration from 10weight percent to the saturation at a temperature of room temperature to100C. Another preferable method is to dip the barium titanate ceramicsin a melt of silver nitrate and/or lithium nitrate for about'lO minutesand to withdraw the barium titanate ceramics from the melt. The coatingcan be achieved by another method to apply a paste including the silvercompound and/or lithium compound in a powder form to the barium titanateceramics.

The barium titanate ceramics having a positive temperature coefficientof electrical resistance can be formed in accordance with any suitableand available ceramic technique as shown in many literature.

The coated barium titanate ceramics are heated at a temperature of 700Cto 1,200C in an oxidizing atmosphere such as air or oxygen. The heatingperiod is dependent upon the heating temperature. The lower heatingtemperature requires the longer heating period. For example, the coatedbarium titanate ceramics is heated at 1,000C for 2 hours. After heating,the remains of coating compound on the surface of barium titanateceramics are mum d- If the silver compound or lithium compound is mixedwith materials of barium titanate ceramics, pressed into a desired formand is sintered, the resultant sintered body does not show the noveleffect according to the present invention, i.e. a higher positivetemperature coefficientof electrical resistance (Rims/R Referring toTable I, compositions 4 to 8, or compositions 9 to 13 shown in Table Ishow the case that silver oxide or lithium carbonate were mixed withother raw materials of barium titanate ceramics of composition 1 ofTable I and calcined, pressed and sintered at l,250C. The ceramics soproduced show a higher resistivities and a lower R u/Rmi value with ahigher amount of silver or lithium carbonate. Thisproves that the bariumtitanate ceramics obtained by a mere addition of silver oxide or lithiumcarbonate into the raw materials is inferior in the electricalproperties to those by the novel method according to the presentinvenhigher than 700C. Operable compounds are silver nition.

Table I Resistivity at C(Jm- Compositions (molar ratio) room temperatureR,,,., ,-R,,,,,

position BaCO TiO, SnO, PbO Nb,0,, Sb O LaiO Ag O Li CO (ohm-cm) 1 1.01.0 .001 40 1O- 2 1.0 .97 .03 .001 45 10 3 .98 1.0 .02 .002 35 40 4 1.01.0 .001 .0001 40 10 5 1.0 1.0 .001 .0001 40 10 6 1.0 1.0 .001 .001 10-7 1.0 1.0 .001 .01 500 10" 8 1.0 1.0 .001 10 10000 10 9 1.0 1.0 .001.00001 40 10 10 1.0 1.0 .001 .0001 40 10" ll 1.0 1.0 .001 .001 10" Tablel C on tinued Resistivity at Compositions (molar ratio) room temperatureR,,,,,,R,,,,,, BaCO; TiO SnO, PbO Nb O Sb O L o. Ag o Li CO; (ohm-cm) 121.0 1.0 .001 .01 600 10 13 1.0 L0 .001 l0 l0000 10 EXAMPLE ture showedtendencies-similar to those of samples 1 to The barium titanate ceramicsin a compound 1 of Table l were prepared by a conventional ceramictechnique. The raw materials corresponding to the composition l arewet-milled and calcined at about 1,000C for 2 hours. The calcinedmixtures were pressed into a desired disc form by a pressure of 700kg/cmand were sintered at 1,350C for 1 hour. The sintered discs were immersedinto the aqueous solution of 20 weight percent of silver nitrate at roomtemperature for 10 minutes to coat with silver nitrate (sample 1 to 6),or into the aqueous solution of 20 weight percent of lithium nitrate atroom temperature for 10 minutes (samples 7 to 12), or into the aqueoussolution containing 10 weight percent of silver nitrate and 10 weightpercent of lithium nitrate at room temperature for 10 minutes (samples13 to 18) as shown in Table II. The barium titanate ceramics were driedand heated at a temperature of 600 to 1,300C. From the results ofsamples 1 to 6, the electrical resistivities of samples 2 to were almostequal to or a little larger than that of the original (composition 1)ceramic subjected to no treatment of silver compound or lithium compoundas shown in Table ll. To the contrary, the raios of R /R became muchlarger by orders of 3/2 to 5/2 than that of the original ceramic. Whensaid coated ceramics were heated above 1,300C, R /R value, for example,of sample l was rather smaller than that of original ceramic, and at thesame time the resistivity of sample I became much larger than that oforiginal ceramic. When said coated ceramics were heated at below 600C,RmaI/R- value, for example, of sample 6 did not become larger than thatof oroginal ceramic.

Samples 7 to 12 treated with silver nitrate aqueous solution and samples13 to 18 treated with silver nitrate and lithium nitrate aqueoussolution at room tempera- Samples 19 to 24, samples 25 to 30 and samples31 to 36 were obtained by immersing the sintered discs in composition 1into a melt of silver nitrate, a melt of lithium nitrate and a melt ofwt percent of silver nitrate and 50 wt percent of lithium nitrate,respectively. So produced samples 19 to 36 showed the tendencies similarto those with samples 1 to 18.

Samples 37 to 48 were obtained by using sintered discs in-composition 2of Table I prepared by a method the same as those of composition 1.Sampes 37 to 42 were immersed into an aqueous solution of 20 wt percentof silver nitrate at room temperature. Samples 43 to 48 were immersedinto a melt of silver nitrate at 300C. The resultant samples 37 to 48showed the similar tendencies to the above mentioned.

Samples 49 to 60 were obtained by using sintered discs in composition 3of Table 1 prepared by a method the same'as that of composition 1.Samples 49 to 54 and samples 55 to were immersed into an aqueoussolution of 20 wt percent of lithium nitrate at room temperature and amelt of lithium nitrate at 300C, respectively, The resultant sinteredsamples 49 to 60 showed the similar tendencies to the above mentioned.

C. A paste of 60 wt percent of lithium carbonate and 40 wt percent ofwater was applied to the samples 67 to 72. The coated discs were heatedat various tem-- peratures ranging from 600C to 1,300C. The resultantsamples 61 to 72 showed the tendencies similar to those o f the abovementioned.

Table II Sample Heating Heating Resistivity Composition Treating vtemperature time R /R No. (C) (hr) (ohm-cm) l immersing 1300 5000 I0" in20 wt% 2 of AgNO 1200 300 10 3 aqueous 1 mo 45 10" solution 4 l at room900 1.0 40 10- 5 tempera- 700 40 I0 ture 6 600 40 10" 7 immersing 13008000 10 in 20 wt% 8 of LiNO I200 400 10" 9 aqueous l 100 45 10 solution10 l at room 900 L0 45 10 l l tem pera- 700 45 10 ture l2 600 40 l0- l3immersing 1300 8000 10"- in aqueous 14 solution I200 500 l0"- l5containing 1 I00 50 l0 10 wt% AgNO lo I and I0 wt% 900 L0 45 l0- l7 LiNOat 700 40 l0' room templ8 erature 600 40 I0 Table II :Qominuei,

Sample Heating Heating Resistivity Composition Treating 1 temperaturetime RMMIRM". No. (C) (hr) (ohmcm) 19 immersing in 1300 7000 10 a meltof v 20 A No, at 1200 400 10 21 300C 1 100 50 10"- 22 1 900 1.0 45 10 13700 45 101.6 24 600 40 10 5 1300 10000 immersing 26 in a melt 1200 50010 27 of LiNO 1 100 50 10 at 300C 28 1 900 1.0 45 10- 29 700 45 0160 30600 40 31 immersing 1300 10000 [O in melt of 32 50 0% of 1200 550 10- 33AgNO; and 50 1100 s0 10 34 1 wt% of LiNO 900 1.0 50 10 at 300C 35 700 451.0 36 600 4.5 3 7 immersing i300 7000 10" in. 20 wt% of 3s AgNO;aqueous 1200 400 10 39 solution at l 100 50 10 room 40 2 tempera- 9001.0 50 10 41 ture 700 I0 42 600 45 10 43 1300 9000 10- 44 immersing in1200 500 l0 a melt of 45 AgNO m 1100 55 10' 46 2 300C 900 1.0 10- 47 70045 v 10" 48 600 45 10- 49 1300 9000 10 50 immersing in I200 400 I0 20wt% Of 5l LiNO; aqueous l mo 45 I0 52 3 3 solution 900 1.0 40 I0 at1'00"] 5 3 tempera- 700 35 10 54 tu re 600 35 I0 55 1300 9000 10'- 56immersing in 1200 500 10 a melt of s7 Li1-1o at 1100 50 10"- 53 3 v 300C900 1.0 40 10 59 7 35 10' 60 600 3s 10 61 1300 7000 10 62 painting 1200350 10 63 with 1 100 45 10 64 1 A ,c0 900 1.0 45 10- 65 700 45 10 66 60040 10 67 1300 7000 10- 68 painting i200 400 10" 69 with v 1 100 50 10 701 Li,c0 900 1.0 45 10 71 700 m 72 600 40 10 wh i lai d i 55 mm oxide,l1th1um oxalate, l1th1um hydroxide and l. A process for improvlng theelectr1cal properties l1th1um n1trate; of a barium titanate ceramic bodyhaving a positive b. heatmg the coated barium titanate ceramic bodytemperature coefficient of electrical res1stance, which at a temperatureof from 700 to 1200C 1n an 0111- comprises: d1z1ng atmosphere; and

a. coating a barium titanate ceramic body having a 60 c. removmg theremains of the coatmg materials on positive temperature coeffic1ent ofelectrical res1sthe surface of the barium titanate ceramlc body. tancewith at least one member selected from the 2. A process according todrum 1, wherein said step group consisting of silver carbonate, silveroxide, of coating 15 carried out by immersing said ceramic silversulphide, silver sulphate, sliver sulphlte, s1lbody 1n an aqueoussolutlon or a melt of s1lver nltrate ver fluoride, silver nitrate,lithium carbonate, lith- 65 and/or lithium nitrate.

' II I! k B

2. A process according to claim 1, wherein said step of coating iscarried out by immersing said ceramic body in an aqueous solution or amelt of silver nitrate and/or lithium nitrate.